It is well known in the art that the light emitted from artificial light sources can change in characteristics depending on a number of factors such as operating temperature and aging of the light source, for example. While technology is improving and light-emitting diodes (LEDs) are being used in increasing numbers of various types of space lighting applications, contemporary high-power LEDs are specifically prone to operating temperature induced colour shifts. Among the different material systems which are used today to implement high-power LEDs which emit various different coloured light, those used for implementing red LEDs are typically most sensitive to changes in temperature. Many multi-colour LED based luminaires therefore require control systems with a form of optical feedback to be able to maintain reasonably stable light emissions. In particular, it is advantageous to measure both the intensity and peak wavelength of light.
Methods and apparatuses for the detection of the light which is emitted by a specific LED or type of LED, for example, in a luminaire under operating conditions, are widely known in the art and readily available. In addition, the operating principles of these devices are described in a number of publications. Many of these solutions, however, suffer from various types of downfalls, most often cost-inefficiency.
For example, U.S. Pat. No. 4,904,088 describes a method and apparatus for determining radiation wavelengths and wavelength-corrected radiation power of monochromatic light sources. It provides an optoelectronic measuring method for determining the wavelength and the wavelength-corrected power of monochromatic light sources. Photodetectors of different spectral overall responsivity are acted upon by the flow of radiation of the light source to be measured through a transfer device. Signals are then produced and transmitted to a calculation unit via a unit for acquiring and processing the measurement values. From the above signals a wavelength-specific quantity is derived in the calculation unit which is compared with the wavelength-specific data present in the memory unit after one calibration. Thus, the actual wavelength of the light source to be measured can be determined, indicated by an indicator unit, or supplied through a data interface. When the actual wavelength is known, a wavelength-specific correction factor can be interrogated in the memory unit, and a wavelength-corrected power can be calculated in the calculation unit. This apparatus for determining radiation wavelengths and wavelength-corrected radiation power is configured for monochromatic light sources and may be complex and potentially cost prohibitive for generic lighting applications.
U.S. Pat. No. 4,309,604 describes a solid state wavelength detection system which can respond to output signals derived from a photoelectric semiconductor device. The photoelectric semiconductor device comprises at least two PN junctions formed at different depths from the surface of the semiconductor substrate. A deeper PN junction develops an output signal related to longer wavelength components of the light impinging thereon. A shallower PN junction develops an output signal related to shorter wavelength components of the impinging light. These two output signals are logarithmically compressed and compared with each other. The difference of the logarithmically compressed output signals represents the wavelength information of the impinging light. The photoelectric semiconductor device however, may be complicated and expensive to fabricate due to the multiple PN junctions and therefore may be cost prohibitive for common applications.
United States Patent Application Publication No. 2004/0022282 describes an arrangement for monitoring the main radiation beam emitted by an optical source such as a laser diode having a nominal emission wavelength. The arrangement includes first and second photodetectors as well as a wavelength selective element. A beam splitter module is provided for splitting a secondary beam from the main radiation beam of the laser source and directing it towards the first photodetector via the associated wavelength selective element. The wavelength selective element has a wavelength selective transmittance-reflectance characteristic, whereby said secondary beam is partly propagated towards said first photodetector and partly reflected from said wavelength selective element towards the second photodetector. The output signals from the photodetectors have intensities whose behaviours are a function of wavelength and are complementary to each other. Signal processing circuitry is further provided including an adder module and a subtractor module fed with the output signals from the photodetectors to generate a wavelength-independent sum signal, indicative of the intensity of the optical radiation generated by the optical source, and a wavelength-dependent difference signal, indicative of the difference between the actual wavelength of the radiation generated by said optical source and its nominal emission wavelength. This arrangement however, may be complicated and cost prohibitive and may not be easily integrated into a lighting device.
Therefore there is a need for a new and cost effective method and apparatus for determining intensities and peak wavelengths of light.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.